A field experiment was conducted in 2017 and 2018 to investigate the responses of fine root growth and morphology plasticity to drip irrigation and nitrogen fertigation (DIF) in a young triploid P. tomentosa plantation in the North China Plain. We monitored four DIF treatments (I20F0, I20FH, I45F0, I45FH), which were the combinations of two irrigation thresholds (− 20 kPa and − 45 kPa, denoted as I20 and I45) and two N fertigation amounts (0, denoted as F0; 220 and 260 kg N ha−1 year−1 in 2017 and 2018, denoted as FH). Some soil properties and aboveground growth parameters were also monitored. I20 significantly improved fine root biomass density (FRBD) by 156.92% and induced a significant reduction of specific root length (SRL) by 28.79% in 0–30 cm soil layers than I45, and FH significantly decreased fine root length density (− 62.88%), surface area (− 54.01%), and SRL (− 39.01%) in 30–80 cm soil layers than F0 in 2017. Fine root traits differed little among DIF treatments in 0–30 cm soil layers in 2018, but I20 significantly improved FRBD by 259.98% than I45 and FH significantly improved fine root averaged diameter by 67.14% in 30–80 cm soil layers than F0. These suggested that fine root had a conservative foraging strategy in water- and nitrogen-rich conditions. All the trait plasticity might be due to the changeable soil properties (i.e. soil water content, organic matter, nitrate and ammonium nitrogen, and total phosphorus). Based on the little difference of fine root mass among DIF treatments in two years, we hold the view that fine root distribution pattern and some growth and morphological traits adapted to water-nitrogen heterogeneity more easily than the mass plasticity in young aged P. tomentosa plantations. Yet enhancing our understanding of fine root foraging strategy in response to DIF practices and how it affects tree growth requires more attention to some physiological and biochemical processes and the growth dynamics of the root systems. • Fine root had a conservative foraging strategy in resource-rich conditions. • Key indices affecting fine root trait plasticity were SWC, OM, NO 3- -N, NH 4+ -N, and TP. • Fine root showed higher trait plasticity compared with mass plasticity under DIF practices. [ABSTRACT FROM AUTHOR]